Lecture 4a: Cameras

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Lecture 4a Cameras
CS6670 Computer Vision
Noah Snavely
Source S. Lazebnik
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Reading

• Szeliski chapter 2.2.3, 2.3

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Image formation

• Lets design a camera
• Idea 1 put a piece of film in front of an
  object
• Do we get a reasonable image?

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Pinhole camera

• Add a barrier to block off most of the rays
• This reduces blurring
• The opening known as the aperture
• How does this transform the image?

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Camera Obscura

• Basic principle known to Mozi (470-390 BC),
  Aristotle (384-322 BC)
• Drawing aid for artists described by Leonardo da
  Vinci (1452-1519)

Gemma Frisius, 1558
Source A. Efros
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Camera Obscura
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Home-made pinhole camera
Why so blurry?
http//www.debevec.org/Pinhole/
Slide by A. Efros
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Shrinking the aperture

• Why not make the aperture as small as possible?

• Less light gets through
• Diffraction effects...

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Shrinking the aperture
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Adding a lens

• A lens focuses light onto the film
• There is a specific distance at which objects are
  in focus
• other points project to a circle of confusion
  in the image
• Changing the shape of the lens changes this
  distance

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Lenses
F
focal point

• A lens focuses parallel rays onto a single focal
  point
• focal point at a distance f beyond the plane of
  the lens (the focal length)
• f is a function of the shape and index of
  refraction of the lens
• Aperture restricts the range of rays
• aperture may be on either side of the lens
• Lenses are typically spherical (easier to produce)

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Thin lenses

• Thin lens equation
• Any object point satisfying this equation is in
  focus
• What is the shape of the focus region?
• How can we change the focus region?
• Thin lens applet http//www.phy.ntnu.edu.tw/java
  /Lens/lens_e.html (by Fu-Kwun Hwang )

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Depth of Field
f / 5.6
f / 32

• Changing the aperture size affects depth of field
• A smaller aperture increases the range in which
  the object is approximately in focus

Flower images from Wikipedia http//en.wikipedia
.org/wiki/Depth_of_field
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Depth of Field
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The eye

• The human eye is a camera
• Iris - colored annulus with radial muscles
• Pupil - the hole (aperture) whose size is
  controlled by the iris
• Whats the film?

• photoreceptor cells (rods and cones) in the retina

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Eyes in nature eyespots to pinhole camera
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Eyes in nature
(polychaete fan worm)
Source Animal Eyes, Land Nilsson
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Before Film was invented
Lens Based Camera Obscura, 1568
Srinivasa Narasimhans slide
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Film camera
Still Life, Louis Jaques Mande Daguerre, 1837
Srinivasa Narasimhans slide
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Silicon Image Detector
Silicon Image Detector, 1970
Shree Nayars slide
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Digital camera

• A digital camera replaces film with a sensor
  array
• Each cell in the array is a Charge Coupled Device
• light-sensitive diode that converts photons to
  electrons
• other variants exist CMOS is becoming more
  popular
• http//electronics.howstuffworks.com/digital-camer
  a.htm

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Color

• So far, weve talked about grayscale images
• What about color?
• Most digital images are comprised of three color
  channels red, green, and, blue which combine
  to create most of the colors we can see
• Why are there three?

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Color perception

• Three types of cones
• Each is sensitive in a different region of the
  spectrum
• but regions overlap
• Short (S) corresponds to blue
• Medium (M) corresponds to green
• Long (L) corresponds to red
• Different sensitivities we are more sensitive
  to green than red
• varies from person to person (and with age)
• Colorblindnessdeficiency in at least one type of
  cone

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Field sequential
YungYu Chuangs slide
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Field sequential
YungYu Chuangs slide
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Field sequential
YungYu Chuangs slide
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Prokudin-Gorskii (early 1900s)
http//www.loc.gov/exhibits/empire/
YungYu Chuangs slide
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Prokudin-Gorskii (early 1990s)
YungYu Chuangs slide
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Color sensing in camera Prism

• Requires three chips and precise alignment
• More expensive

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Color filter array
Bayer grid
Estimate missing components from neighboring
values(demosaicing)
Why more green?
Source Steve Seitz
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Bayers pattern
YungYu Chuangs slide
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Foveon X3 sensor

• Light penetrates to different depths for
  different wavelengths
• Multilayer CMOS sensor gets 3 different spectral
  sensitivities

YungYu Chuangs slide
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Color filter array
red
green
blue
output
YungYu Chuangs slide
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X3 technology
red
green
blue
output
YungYu Chuangs slide
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Foveon X3 sensor
X3 sensor
Bayer CFA
YungYu Chuangs slide
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Historical context

• Pinhole model Mozi (470-390 BC), Aristotle
  (384-322 BC)
• Principles of optics (including lenses) Alhacen
  (965-1039)
• Camera obscura Leonardo da Vinci (1452-1519),
  Johann Zahn (1631-1707)
• First photo Joseph Nicephore Niepce (1822)
• Daguerréotypes (1839)
• Photographic film (Eastman, 1889)
• Cinema (Lumière Brothers, 1895)
• Color Photography (Lumière Brothers, 1908)
• Television (Baird, Farnsworth, Zworykin, 1920s)
• First consumer camera with CCD Sony Mavica
  (1981)
• First fully digital camera Kodak DCS100 (1990)

Alhacens notes
Niepce, La Table Servie, 1822
CCD chip